The ovarian steroid hormones, estrogen and progesterone, promote accumulation of genetic changes during the development of breast cancers, and drive the growth of established breast cancer. During late stages of breast cancer progression, tumors usually become steroid hormone resistant and concurrent amplification of type I (EGFR/c-erbB2) tyrosine kinase growth factor receptors occurs with high frequency; these alterations are associated with a particularly poor prognosis. EGF/erbB2 receptor signalling initiates cytoplasmic protein kinase phosphorylation events that are part of the mitogen-activated protein (MAP) kinase cascade. Endpoints of MAPK signalling include activation of transcription factors that control the expression of genes involved in cell growth regulation. In breast cancer cells, EGFR and c-erbB2 receptors are regulated by steroid hormones, and cross-talk between these pathways is becoming well-established. Phosphorylation of progesterone receptors (PR) by MAPK is required for ligand-dependent downregulation by the ubiquitin-proteasome pathway. Paradoxically, this same event mediates nuclear association of PR and greatly potentiates PR transcriptional activity. Our preliminary data show that growth factors are key regulators of steroid hormone sensitivity. We postulate that growth factors drive breast cancer cell growth in part by "de-repressing" steroid hormone receptor function. This involves regulatory interplay between receptor phosphorylation, ubiquitination, and sumoylation. The specific aims in this proposal will address the following questions: 1) Which kinase pathways input to PR regulation? 2) What is the basis for the inverse relationship between PR stability and transcriptional activity? And 3) What is the functional significance of PR sumoylation in response to growth factor and steroid hormone signalling? These aims will define the molecular mechanisms of growth factor regulation of PR function in breast cancer cells. The long-term goal is to define the role of growth factor signalling in the progression of breast cancer cells towards steroid hormone resistance. If we can understand the mechanisms that control this switch from growth stimulation by steroid hormones to growth factors, this pathway could, in theory be blocked, which would lead to longer disease-free survival of patients with breast cancer. This work will aid our understanding of how steroid hormone resistance develops, and will identify new therapeutic targets for breast cancer treatment.